This invention relates to the measurement of coefficients of friction on walking surfaces and more specifically concerns an improvement in a widely used type of instrument known as a Horizontal Pull Slipmeter, hereinafter referred to by the acronym HPS.
The HPS is generally accepted by industrial standards as a device for measuring the coefficient of friction of a walking surface. It was developed by C. H. Irvine of Liberty Mutual Insurance Co. of Hopkinton, Masss., and is described in ASTM Method F:609-79. Its standard construction consists of two basic elements: (a) a drag sled mounted upon slider feet and fitted with a force gage, and (b) means for pulling the sled across the floor surface being tested at a controlled rate of speed. Although the design of the drag sled and force gage are specified in industry standards, the configuration of the pulling means may take any form, provided it serves the specified function. In most applications, a servo-type unit is employed to pull the sled in a horizontal manner across a floor surface by means of a flexible string which is wound onto a wheel driven at a prescribed constant speed by the servo unit.
The HPS has specific attributes that yield advantages over alternative devices for making such measurements. It is portable, simple to use, and self-calibrating. However, the device has certain shortcomings which limit its further acceptance as an industrial standard tester. The HPS will not accurately measure the coefficient of friction (COF) on wet surfaces because it takes too long to set up the two components for testing. By the time both components are placed into position and connected by means of the string, the slider feet of the drag sled have displaced most of the moisture from the surface being tested. Furthermore, the time delay permits the resilient material of the slider feet to begin to conform to the surface microtopography (a phenomena known as the "sticksion problem"). These factors can contribute to artificially high friction readings. In addition, vagaries in the arrangement of pulling means employed in the test may introduce an additonal variable and the possibility of operator-dependent bias in results. For example, even though the wheel which winds the string is driven at a constant speed, the details of alignment are not controlled.
It is therefore an object of the present invention to provide a device capable of taking valid coefficient of friction measurements on wet or dry surfaces.
It is another object to provide a device of the aforesaid nature having no operator-dependent variables.
It is yet another object of the invention to provide a device of the aforesaid nature which is faster and easier to use than the present HPS instrument.